1. Field of the Invention
The present invention relates to an image scanning device having a dual-side scanning function to scan both sides of a document through one transportation such as a digital copy machine, facsimile, and scanner as well as, an image scanning method, an image formation apparatus, and a recording medium.
2. Description of the Related Art
An image scanning device employed as a copy machine or the like is conventionally known to carry out the so-called “skim reading”, i.e. scanning an image on a document, having the scanning position by a sensor such as a CCD (Charge Coupled Device) fixed, and delivering a document onto a platen glass, one page at a time, through an automatic document reader, and reading out the light reflected from the document at the scanning position. There is also known a “dual-side scanning” type having two sensors (such as CCD, CIS (Contact Image Sensor)) provided to scan the top side and back side of a document through one transportation to improve the production output, as disclosed in Japanese Laid-Open Patent Publication No. 2006-115134, for example.
FIG. 24 is a diagram to describe a general configuration of an image scanning device capable of dual-side scanning.
Referring to FIG. 24, there is established a transportation path 1220 through which a document is transported inside the image scanning device. A plurality of transportation rollers are provided to carry a document through the transportation path.
Specifically, there are sequentially provided a read roller pair 1119, a read discharge roller pair 1121, and a discharge sheet roller pair 1123 from the upstream side in the document transportation direction of the transportation path. A scanning position Q1 of CCD 1120 is located between read roller pair 1119 and read discharge roller pair 1121. A scanning position Q2 of CIS 1129 is located between read discharge roller pair 1121 and discharge roller pair 1123.
A document carried through transportation path 1220 has the surface (top side) scanned by CCD 1120 at scanning position Q1 and the rear surface (back side) of the document scanned by CIS 1129 at scanning position Q2. Thus, dual-side scanning is executed by scanning both the top side and back side of a document through one transportation.
In order to prevent slacking in the document at the time of passing the scanning position, an image scanning device performing skim reading generally has the document transportation speed by the transportation roller disposed downstream of the scanning position set higher (faster) than the transportation speed by the transportation roller disposed upstream of the scanning position.
The transportation speed by the transportation rollers arranged at the upstream and downstream sides of the scanning position cannot be made to coincide with each other completely due to variation in the fabrication of the transportation rollers, wear of the transportation rollers due to usage, and the like. In order to prevent slacking in a document caused by the transportation speed by the transportation roller located downstream being lower (slower) than the transportation speed by the transportation roller located upstream, the transportation speed of the transportation roller located downstream of the scanning position is designed to be higher than the transportation speed by the transportation roller disposed upstream of the scanning position.
Slacking in the document at the scanning position will lead to degradation in the resolution of the scanned image (occurrence of out-of-focus).
Thus, in the image scanning device of FIG. 24, the transportation speed of a document by read discharge roller pair 1121 located downstream of scanning position Q1 is higher than the transportation speed by read roller pair 1119 located upstream of scanning position Q1 of CCD 1120.
Accordingly, the document will be constantly pulled towards the downstream side at scanning position Q1, whereby slacking in document at scanning position Q1 can be prevented.
Furthermore, the transportation speed of a document by discharge roller pair 1123 located downstream of scanning position Q2 is set higher than the transportation speed of a document by read discharge roller pair 1121 located upstream of scanning position Q2 of CIS 11129. Accordingly, the document will be constantly pulled towards the downstream side also at reader position Q2 to prevent slacking in the document at scanning position Q2.
When the transportation speed by the transportation roller located downstream in the transportation path is set higher than the transportation speed by the transportation roller located upstream in the transportation path, the transportation speed of a document at scanning position Q1 of CCD 1120 will differ from the transportation speed of the document at scanning position P2 of CIS 1129. As a result, in the event of dual-side scanning, there is a problem that the magnification of the scanned image from the topside of the document read out by CCD 1120 will differ from the magnification of the scanned image from the back side of the document read out by CIS 1129.
Specifically, there is a problem that the magnification in the subscanning direction of the scanned image from the back side of the document will become smaller with respect to the magnification in the subscanning direction of the scanned image from the back side of the document.
The aforementioned Japanese Laid-Open Patent Publication No. 2006-115134 does not take into account the fact that the document transportation speed of a document by the transportation roller located at the downstream side in the transportation path is set higher than the transportation speed by the transportation roller located at the upstream side in the transportation path, and cannot address degradation in the resolution caused by slacking in a document at the scanning position.
Japanese Laid-Open Patent Publication No. 2008-104165 discloses a system of correcting the magnification in the subscanning direction by increasing the transportation speed by the roller located at the downstream side higher than the transportation speed by the roller located at the upstream side, and modifying the scanning period in the main scanning direction between the top side and back side of a document (scanning time per one line). Specifically, there is disclosed a system of modifying the scanning period in the main scanning direction using a pulse generator.
The magnification correction system disclosed in the aforementioned Japanese Laid-Open Patent Publication No. 2008-104165 locally modifies the scanning period in the main scanning direction during document scanning. Therefore, the timing control is complicated, and a particular hardware configuration to realize such control is required.
Furthermore, there is a problem of degradation in the image quality. Since the scanning period per one line is modified locally, density variation is generated in the image scanned from the document at the region where the scanning time is switched.
There is also known another magnification correction system that temporarily stores the scanned image of the back side in a memory, and applying a predetermined image processing algorithm on the data to increase the number of lines in the back side scanned image (inflate processing).
However, this system has the problem that the advantage of dual-side scanning for image reading at high speed is degraded since the data of the back side scanned image is temporarily stored in a memory, which had to be read out again to be subjected to image processing.